Heating device employing thermistor with positive coefficient characteristic

ABSTRACT

A heating device for heating fluid is disclosed. The heating device includes a tanning unit including a thermistor element having a positive temperature coefficient characteristic and adapted to generate heat when electric power is applied thereto. At least one heat dissipating means having a plurality of through-holes defined therein is mounted on the heating unit in thermal conductive relation with the heating unit such that heat generated by the heating unit is transmitted to the heat dissipating means to heat fluid flowing through the through-holes. The thermistor element includes a pair of electrodes between which heating current flows. The location of the electrodes is chosen such that the current flows in a second direction, substantially perpendicular to the first direction.

BACKGROUND OF THE INVENTION

The present invention relates to a heating device for heating fluid,such as air, and more particularly, to an improvement in the arrangementof the heating device employing, for the source of heat, ceramics, suchas a thermistor having a positive temperature coefficientcharacteristic.

One conventional heating device is shown in FIG. 1 in which a ceramicblock 1 having a plurality of through-holes 2 formed in the directionparallel to the direction of thickness of the ceramic block 1 isdisposed in the path of flow of air generated by the fan 5. The ceramicblock 1 has first and second electrodes 3 and 4 on the opposite flatsurfaces thereof except on the openings of the through-holes 2. When thevoltage is applied between the electrodes 3 and 4, an electric currentflows through the ceramic block 1 in the direction of thickness thereofand, as a result, heat is generated from the ceramic block 1 and isradiated or released to the surrounding atmosphere. During the supply ofthe voltage to the ceramic block 1 and when the air is not flowingthrough the through-holes 2, the temperature of the block 1 rises by thegreatest amount at the center portion between the electrodes 3 and 4,and by a gradually decreasing amount towards the opposite surfacesprovided with the electrodes 3 and 4. This temperature distributionalong the direction of thickness of the ceramic block 1 is shown by acurve W0 in FIG. 2. When the fan 5 is driven to generate wind W in thedirection shown by the arrows of FIG. 2, the heat in the ceramic block1, particularly in an intake region A-B close to the surface of theblock which confronts the coming air, is released to the air for heatingthe air passing through the through-holes 2. As a consequence, thetemperature in the intake region A-B is reduced, thus shifting thetemperature peak towards an outlet region B-C located close to the othersurface of the block 1. Therefore, the temperature distribution alongthe direction of thickness of the ceramic block 1 under the abovecondition results in a curve W1 shown in FIG. 2.

Since the material constituting the block 1 has a positive temperaturecoefficient characteristic, its resistance increases with increasedtemperature. Therefore, when the temperature distribution along thethickness direction of the ceramic block 1 corresponds to the curve W1,the resistance in the outlet region B-C becomes considerably higher thanthe resistance in the intake region A-B. Since the direction of electriccurrent flow through the ceramic block 1 is in alignment with thethickness direction of the block 1, i.e., the direction of air flow, theelectric resistance change in the outlet region B-C strongly influencesthe amount of electric current flow through the intake region A-B.Accordingly, the conventional heating device has such a disadvantagethat the electric current flowing through the block 1 between A and C(FIG. 2) is undesirably limited by the high resistance in the outletregion B-C, causing a so-called pinch effect. Therefore, the heatgeneration is effected more efficiently in the outlet region B-C than inthe intake region A-B where the heat release from the block 1 to the airis effected eminently. Thus, as a whole, the conventional heating deviceheat radiation efficiency.

Furthermore, since the ceramic block 1 directly touches, and releasesthe heat to, the incoming air, the conventional heating device has theadditional disadvantage that the heat generated from the ceramic block 1may become unstable particularly when the wind velocity increasesabruptly, as explained below.

Generally, when the wind velocity increases, more heat is released fromthe ceramic block 1 to the passing air, causing a temperature drop inthe ceramic block 1. This temperature drop results in the decrease ofthe resistance of the block 1. Thus, the current flowing through theblock 1 increases to enhance the heat generation. However, if the windvelocity is increased abruptly as often caused by the change in thespeed of the fan 5, the temperature drop in the ceramic block 1 isinstantaneously dropped to instantaneously decrease the resistance ofthe block 1, causing a rapid increase of the current flowing through theblock 1. This rapid increase of the current enhances the heat generationto rise the temperature of the block 1 above the temperature at whichthe ceramic block 1 loses its positive temperature coefficientcharacteristics (i.e. exhibits a negative temperature coefficientcharacteristic) and, as a result, the resistance of the block 1 becomesunstable. Thus, the power consumed in the ceramic block 1 may beundesirably oscillated causing an undesirable fluctuation intemperature.

BRIEF DESCRIPTION OF THE INVENTION

Accordingly, it is a primary object of the present invention to providean improved heating device employing ceramics having a positivetemperature coefficient characteristic in which the electric currentflowing through the ceramics in the intake region is independent of theelectric current flowing through the ceramics in the outlet region.

It is another object of the present invention to provide a heatingdevice of the above described type in which the heat transfer from theheat generating ceramics to the incoming fluid is effected graduallyregardless of the abrupt change in the velocity of incoming fluid.

In order to accomplish these objects, a heating device of the presentinvention provides an independent current path for each of the intakeand outlet regions. According to this arrangement, the resistance changein the outlet region does not strongly influence the current flowthrough the inlet region. In other words, the current flow through theceramics will not be strongly influenced by the temperature distributionalong the ceramics in the direction parallel to the direction of flow offluid.

The heating device of the present invention includes a heat dissipatingblock and a heat generating ceramics which are tightly attached togetherfor the heat flow from the heat generating ceramics to the heatdissipating block. Since the fluid to be heated flows through the heatdissipating block, the abrupt change in the velocity of the incomingfluid will not result in an abrupt temperature drop of the heatgenerating ceramics.

In accordance with a preferred embodiment of the invention, a heatingdevice for heating fluid comprises a heat dissipating means having aplurality of through-holes defined therein and a heating unit includinga thermistor element having a positive temperature coefficientcharacteristic. The heating unit is adopted to generate heat whenelectrical power is applied thereto. The heat dissipating means and theheating unit are mounted in a thermal conductive relation to each othersuch that the heat generated by the heating unit is transmitted to theheat dissipating means to heat the fluid flowing through thethrough-holes. The heating device further comprises means for connectingthe heat dissipating means and the heating unit together.

According to another preferable embodiment of the invention, a heatingdevice comprises a heat dissipating block having first and second facesopposed to each other, a side face extending between respective edges ofthe first and second faces, and a plurality of through-holes extendingbetween the first and second faces in parallel to each other for thepassage of the fluid therethrough. The heat dissipating block includesan intake region adjacent to the first face and an outlet regionadjacent to the second face. A heating unit includes a thermistor platehaving a positive temperature coefficient characteristics, and first andsecond electrodes deposited on the thermistor plate. The heating unit isheld in contact with a portion of the side face of the heat dissipatingblock by a suitable supporting means. The heating unit has first andsecond regions which are respectively attached to the intake and outletregions of the heat dissipating block. The first and second electrodesare adopted to provide an electric current to flow parallelly throughthe first and second regions of the thermistor plate for generating heatfrom the first and second regions. The heating device further comprisesmeans for connecting the heat dissipating block and the heating unit.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and features of the present invention willbecome apparent from the following description taken in conjunction withpreferred embodiments thereof with reference to the accompanyingdrawings, in which:

FIGS. 1 and 2 are drawings which have been already referred to in theforegoing description, FIG. 1 being a diagrammatic view of a heatingdevice according to the prior art, and FIG. 2 being a graph showing atemperature distribution along the thickness direction of the heatgenerating block;

FIG. 3 is a perspective view of a heating device according to the firstembodiment of the present invention;

FIG. 4 is a cross-sectional view taken along the line IV--IV of FIG. 3;

FIG. 5 is a perspective view of a thermistor employed in the device ofFIG. 3;

FIG. 6 is a front view partly broken of a heat dissipating block;

FIG. 7 is a view similar to FIG. 6, but particularly shows amodification thereof;

FIG. 8 is a perspective view of a frame employed in the heating deviceof FIG. 7;

FIG. 9 is a view similar to FIG. 6, but particularly shows anothermodification thereof;

FIG. 10 is a view similar to FIG. 6, but particularly shows a furthermodification thereof;

FIG. 11 is a front view partly broken of a heating device according tothe second embodiment of the present invention;

FIG. 12 is a perspective view of a thermistor employed in the heatingdevice of FIG. 11;

FIG. 13 is a perspective view showing a modification of the thermistorshown in FIG. 12;

FIG. 14 is a perspective view of the thermistor of FIG. 13 viewed fromanother angle;

FIG. 15 is a cross-sectional view of a heating device according to thethird embodiment of the present invention;

FIG. 16 is a front view partly broken of a heating device which is amodification of the heating device shown in FIG. 15;

FIG. 17 is a frame employed in the heating device of FIG. 16;

FIG. 18 is a cross-sectional view of a supporting member employed in theheating device of FIG. 16;

FIG. 19 is an enlarged fragmentary view of the heating device shown inFIG. 16;

FIG. 20 is a schematic view showing an electrical connection to the heatdissipating block of FIG. 19;

FIG. 21 is a fragmentary view showing a modification of the supportingmember of FIG. 18;

FIG. 22 is a fragmentary sectional view showing a condition in which afurther modified supporting member of FIG. 23 is mounted on flanges ofheat dissipating blocks;

FIG. 23 is a perspective view of the further modified supporting member;

FIG. 24 is an enlarged fragmentary view of a modification of the heatingdevice shown in FIG. 16;

FIG. 25 is a schematic view showing an electrical connection to the heatdissipating block of FIG. 24;

FIG. 26 is a perspective view showing a modification of the heatingdevice of FIG. 15;

FIG. 27 is an exploded view of the heating device of FIG. 26;

FIG. 28 is a view similar to FIG. 26, but particularly showing amodification thereof;

FIG. 29 is a schematic view showing a manner in which the heatingdevices of FIG. 28 are connected;

FIG. 30 is a front view partly broken of a heating device which is afurther modification of the heating device shown in FIG. 15;

FIG. 31 is an exploded view of the heating device of FIG. 30;

FIG. 32 is a perspective view of frame employed in the heating device ofFIG. 30;

FIG. 33 is a perspective view showing a modification of the frame ofFIG. 32;

FIG. 34 is a front view partly broken of a heating device which is a yetanother modification of the heating device shown in FIG. 15;

FIG. 35 is a perspective view of a supporting plate employed in theheating device of FIG. 34;

FIG. 36 is a fragmentary sectional view showing an engagement betweenthe supporting plate of FIG. 35 and an elongated plate;

FIG. 37 is a fragmentary view showing a manner in which the heatdissipating block is supported on the supporting plate;

FIG. 38 is a sectional view of a heating device which is a still furthermodification of the heating device shown in FIG. 15; and

FIGS. 39 to 42 are front views of a heat dissipating block showingdifferent patterns of through-holes.

In the following description of the invention, several embodiments ofthe present invention will be described individually under therespective headings. Modification or modifications of each embodimentwill be described under the respective sub-headings following thedescription of the relevant embodiment. It is to be noted that likeparts in each embodiment are designated by like reference numeralsthroughout the drawings.

EMBODIMENT 1

Referring to FIGS. 3 and 4, a heating device of this embodimentcomprises two heat dissipating blocks 101 and 102, each having abox-like configuration and a plurality of through-holes 103 of hexagonalcross-section formed therein in a substantially honeycomb-like pattern.The through-holes 103 extend in parallel to each other and also to thedirection of thickness of the corresponding blocks. Each of the blocks101 and 102 is made of a material having a high heat conductivity and ahigh electric conductivity, such as aluminum or copper.

A heat generating unit 104, such as a thermistor, is tightly heldbetween the blocks 101 and 102. The heat generating unit 104 comprises,as best shown in FIG. 5, a rectangular plate 104a made of a material(such as ceramics mainly consisting of barium titanate) having apositive temperature coefficient characteristic, and first and secondelectrodes 104b and 104c deposited on the opposite flat surfaces of theplate 104a in ohmic contact therewith. The size of the thermistor 104 isapproximately equal to that of one side surface of the block 101 or 102so that, when the thermistor 104 is sandwiched between the blocks 101and 102, all the side faces of the thermistor 104 are flush with theside faces of the blocks 101 and 102.

The heat dissipating block 101 has a pair of flanges 101a and 101bprotruding laterally outwards therefrom flush with the surface of theblock 101 in contact with the heat generating unit 104. The flanges 101aand 101b have openings 101c and 101d, respectively, formed therein forreceiving a fastener comprising a bolt and nut therethrough in a mannerwhich will be described below. It is to be noted that the opening 101dformed in the flange 101b is larger than the opening 101c formed in theflange 101a. Similarly, the heat dissipating block 102 has a pair offlanges 102a and 102b protruding laterally outwards therefrom flush withthe surface of the block 102 in contact with the heat generating unit104. The flanges 102a and 102b have openings 102c and 102d,respectively, the opening 102d in the flange 102b being smaller than theopening 102c in the flange 102a.

After the thermistor 104 has been held between the blocks 101 and 102 inthe manner described above, the flanges 101a and 102a are interconnectedwith each other by the use of a set of bolt 105 and nut 106. Since theblocks 101 and 102 are made of an electric conductive material, a rubberwasher 107 having a ring portion and a cylindrical body portion isinserted in the opening 102c in the flange 102a to electrically insulatethe bolt 105 and nut 106 from the block 102. For this purpose, theopening 102c in the flange 102b is larger than that of the flange 102a.A terminal tab 108 is mounted on the bolt 105 and held in contact withthe flange 101a for the external electrical connection. It is preferableto mount a metal washer 109 between the nut 106 and the rubber washer107. Likewise, the flanges 101b and 102b are interconnected with eachother by the use of a fastener comprising a bolt 110 and a nut 111. Inthis case, a rubber washer 112 is inserted in the opening 101d in theflange 101b, while a terminal tab 113 is mounted on the bolt 110 incontact with the flange 102b. Preferably, a metal washer is mounted onthe bolt 110 between the flange 102b and the nut 111.

When the electric power from a suitable power source (not shown) isapplied between the terminals 108 and 113, the potential at the terminal108 is transmitted through the bolt 105 and the heat dissipating block101 to the electrode 104b which is held in contact with the block 101,whereas the potential at the terminal 113 is transmitted through thebolt 110 and the heat dissipating block 102 to the electrode 104c whichis held in contact with the block 102. When the voltage from the powersource is so fed to the thermistor 104 in the manner described above, anelectric current flows through the ceramic plate 104a in the directionof thickness thereof and, as a result, heat is generated in the ceramicplate 104a. The generated heat is transmitted to the heat dissipatingblocks 101 and 102 to heat the latter. Since the blocks 101 and 102 aredisposed in the path of flow of fluid, such as air, with thethrough-holes 103 in alignment with the direction of air flow W (FIG.3), the air passing through the through-holes 103 is heated.

In the heating device described above, since the direction of currentflow through the ceramic plate 104a is not in alignment with thedirection of air flow but in perpendicular relation to the air flow, theelectric current will not be strongly influenced by the temperaturedistribution in the thermistor. In other words, since the currentflowing through an intake region A-B (FIG. 3) of the thermistor 104located close to the surface of the heat dissipating blocks 101 and 102confronting the incoming air is parallel to the current flowing throughan outlet region B-C (FIG. 3) of the thermistor 104 located close to thesurface of the heat dissipating blocks 101 and 102 opposite to the abovementioned surface, the heat generation in the intake region A-B iscarried out by an electric current which is independent of the currentflowing through the outlet region B-C. Therefore, the resistance changein the outlet region B-C caused by the temperature change in that regionB-C will not strongly affect the current flowing through the intakeregion A-B. Therefore, there will be no pinch effect produced in theceramic plate 104a. Thus, the air passing through the through-holes 103can be heated with high efficiency.

Furthermore, since the heat generated from the thermistor 104 is firsttransmitted to the heat dissipating blocks 101 and 102 to avoid thedirect contact of the air to the thermistor 104, the abrupt change inthe velocity of the air flow will not result in the abrupt change in thetemperature of the thermistor 104. Therefore, no electric poweroscillation will be produced.

MODIFICATION 1

Referring to FIG. 6, each of the heat dissipating blocks 101 and 102 canbe formed with a recess 115 in the surface which is held in contact withthe thermistor 104 to accommodate the thermistor 104 therein forpreventing the thermistor 104 from being displaced.

MODIFICATION 2

Referring to FIGS. 7 and 8, instead of forming the recess 115 as shownin FIG. 6, the heating device of this Modification 2 is further providedwith a frame 116 made of non-conductive material for supporting thethermistor 104 in place between the blocks 101 and 102. The frame 116has a large rectangular opening 116a in the center for receiving thethermistor 104 and two small circular openings 116b and 116c in theopposite side beams for receiving the bolts 105 and 110, respectively.It is preferable to arrange the thickness of the frame 116 slightlythinner than the thickness of the thermistor 104 for effecting a tightcontact between the thermistor 104 and blocks 101 and 102.

MODIFICATION 3

Referring to FIG. 9, the thermistor 104 which has been shown anddescribed as being formed by one unit can be formed by two or moreunits, such as four thermistors 104W, 104X, 104Y and 104Z, as shown. Inthis case, it is preferable to provide a predetermined gap 117 betweentwo neighboring thermistors for allowing air to pass therethrough,resulting in an effective heat transmission from each of the thermistorsto the passing air. Furthermore, the presentation of the gap increasesthe path of air, thus increasing the amount of air passing through theheating device per a unit time.

MODIFICATION 4

Referring to FIG. 10, the heating device of this modification has twothermistors 104W and 104X which are positioned side-by-side with apredetermined gap 117 therebetween, each thermistor 104W or 104X havingceramic plate 104a and electrodes 104b and 104c. The heating devicefurther has a pair of common electrode plates 118 and 119 bonded to theelectrodes 104b and 104c, respectively, of the thermistors 104W and 104Xby the use of electrically conductive bonding agent. These commonelectrode plates 118 and 119 has tabs 118a and 119a, respectively, forthe external electric connection thereto. The heating device further hasa pair of insulation layers 120 and 121 made of a high heat conductivematerial, such as aluminous porcelain and positioned between the commonelectrode plate 118 and the heat dissipating block 101 and between thecommon electrode plate 119 and the heat dissipating block 102,respectively. According to this modification, the insulation layers 120and 121 are held in position by the use of bonding agent so that in thiscase, it is not necessary to provide flanges and sets of bolt and nutfor interconnecting the blocks together.

According to this embodiment, since the heat dissipating blocks 101 and102 are electrically insulated from the thermistors 104W and 104X, theycan be disposed in the path of electrically conductive fluid, such aswater. In the case where the heating device is to be entirely disposedin the path of electrically conductive fluid, it is necessary to shieldthe thermistors 104W and 104X by any known method.

EMBODIMENT 2

Referring to FIG. 11, a heating device of this embodiment comprises oneheat dissipating block 201 formed with a plurality of through-holes 202in the same manner as the heat dissipating block 101 described above inthe Embodiment 1 with reference to FIGS. 3 and 4.

A sheet 203 made of an insulating material and having a high heatconductivity, such as aluminous porcelain, is tightly deposited on oneflat surface of the heat dissipating block 201 by the use of bondingagent.

A heat generating unit 204, such as a thermistor, includes, as shown inFIG. 12, a rectangular plate 204a made of ceramics having a positivetemperature coefficient characteristics and first and second electrodes204b and 204c deposited on one flat surface of the ceramic plate 204a ina side-by-side relation to each other and in an ohmic contact with theflat surface. Terminal legs 205a and 205b are connected to theelectrodes 204b and 204c by the deposition of solder beads 206a and206b, respectively. For facilitating the soldering, the end portion ofeach of terminal legs 205a and 205b connected to the electrodes 204a and204b, respectively, is bent at right angles. The other end portion ofeach of terminal legs 205a and 205b is formed with openings 207a or 207bfor facilitating the external connection thereto. The flat surface ofthe rectangular plate 204a opposite to the surface provided with theelectrodes 204b and 204c is attached to the sheet 203 in such a mannerthat the terminal legs 205a and 205b are aligned in a directionperpendicular to the through-holes 202. The attachment of the heatingblock to the sheet 203 can be effected by the use of bonding agent.

When the voltage from a suitable power source (not shown) is appliedbetween the terminal legs 205a and 205b, an electric current flowsthrough the ceramic plate 204a for generating heat therefrom. Thegenerated heat is transmitted to the heat dissipating block 201 throughthe insulation sheet 203. Since the terminal legs 205a and 205b arealigned perpendicular to the through-holes 202, the direction of flow ofelectric current through the ceramic plate 204a is in perpendicularrelation to the air flow through the through-holes 202. Accordingly, theelectric current will not be strongly influenced by the temperaturedistribution in the thermistor.

Furthermore, since the heat generated from the thermistor 204 istransmitted to the air through the heat dissipating block 201, theabrupt change in the velocity of the air flow will not result in theabrupt change in the temperature of the thermistor 204. Therefore, noelectric power oscillation will be produced.

MODIFICATION 1

Referring to FIGS. 13 and 14, there is shown a modified thermistor 208comprising a rectangular plate 208a made of ceramics having a positivetemperature coefficient characteristics and a pair of comb-likeelectrodes 208b and 208c which are interleaving with each other anddeposited on one flat surface of the ceramic plate 208a. The comb-likeelectrode 208a extends along the side to the opposite flat surface ofthe ceramic plate 208a. The terminal leg 205a is soldered to theelectrode 208b at the above mentioned opposite flat surface of theceramic plate 208a in a similar manner described above with reference toFIGS. 11 and 12. Similarly, the comb-like electrode 208c extends alongthe side to the opposite flat surface of the ceramic plate 208a forsoldering the terminal leg 205b thereto. The surface of the thermistor208 provided with the interleaving electrodes 208b and 208c is bonded tothe sheet 203 in such a manner that the direction of teeth of thecomb-like electrodes 208b and 208c is in alignment with thethrough-holes 202. Accordingly, when the voltage is applied between theterminal legs 205a and 205b, electric current flows through the ceramicplate 208a in the perpendicular direction to the air flow.

Although the teeth of the comb-like electrodes 208b and 208c have beendescribed as extending in parallel to the through-holes 202, it ispossible to align the teeth in any other direction because, when theinterleaving electrodes are employed, the distance of the current flowthrough the ceramic plate 208a between the teeth is much shorter thanthe widthwise direction of the heat dissipating block 201 in which thetemperature distribution discussed above appears.

EMBODIMENT 3

Referring to FIG. 15, a heating device of this embodiment comprisesthree heat dissipating blocks 301, 302 and 303, each having a box-likeconfiguration and a plurality of through-holes 304 of hexagonalcross-section formed therein in a substantially honeycomb-like pattern.The through-holes 304 extend in parallel to each other and also parallelto the direction of thickness of the corresponding blocks. Each of theblocks 301, 302 and 303 is made of a material having a high heatconductivity and a high electric conductivity, such as aluminum orcopper.

A heat generating unit 305, such as a thermistor, is tightly heldbetween the blocks 301 and 302, and another heat generating unit 306 istightly held between the blocks 302 and 303 in the manner describedabove with reference to FIG. 4. Each of the heat generating units 305and 306 has the same structure as the heat generating unit 104 describedabove with reference to FIG. 5. More particularly, the heat generatingunit 305 is constituted of a ceramic plate 305a having a positivetemperature coefficient characteristic and electrodes 305b and 305cdeposited on opposite flat surfaces of the ceramic plate 305a. Likewise,the heat generating unit 305 is constituted of a ceramic plate 306a andelectrodes 306b and 306c.

The heat dissipating block 301 has a pair of flanges 301a and 301bprotruding laterally outwards therefrom and flush with the surface ofthe block 301 in contact with the heat generating unit 305. Similarly,the heat dissipating block 303 has a pair of flanges 303a and 303bprotruding outwards therefrom. The heat dissipating block 302 has a pairof flanges 302a and 302b protruding laterally outwards therefromapproximately at the center portion between the surfaces held in contactwith the heat generating units 305 and 306. Each of the flanges 301a,302a and 303a has an opening formed therein for receiving a set of bolt307a and a nut 307b, while each of the flanges 301b, 302b and 303b hasan opening for receiving another set of bolt 308a and 308b to tightlyhold the thermistors 305 and 306 between the blocks. When mounting thenuts 307b and 308b on the bolts 307a and 307b, respectively, it ispreferable to put a washer between the nut and flange. The bolts 307aand 308a are also provided for the purpose of electric connectionbetween the heat dissipating blocks 301 and 303. For this purpose, atube 309 made of an insulating material is mounted on the bolt 307abetween the flanges 301a and 303a to avoid any electrical connectionbetween the bolt 307a and the center block 302. Similarly, a tube 310 ofan insulating material is mounted on the bolt 308a between the flanges301b and 303b. A terminal tab 311 is mounted on the bolt 307a and heldin contact with the flange 301a for the external electrical connectionto the blocks 301 and 303. The electrical connection to the center block302 is carried out by a terminal tab 312 connected to the flange 302a bya screw 313 or any other connecting means, such as soldering.

According to a preferable arrangement, the height H1 of the center heatdissipating block 302 is greater than the height H2 of the heatdissipating blocks 301 and 303 to balance the transfer of heat from theheat generating units 305 and 306 to the blocks 301, 302 and 303.

When the electric power is applied between terminals 311 and 312, anelectric current flows through the ceramic plates 305a and 306a causingthem to generate heat. The generated heat is transmitted to the heatdissipating blocks 301, 302 and 303. Since the electric current flowingthrough each of the ceramic plates 305a and 306a is in perpendicularrelation to the air flow through the through-holes 304, the electriccurrent will not be strongly influenced by the temperature distributionin the thermistors 305 and 306.

Furthermore, since the heat generated from the thermistors 305 and 306is transmitted to the air through the heat dissipating blocks 301, 302and 303, the abrupt change in the velocity of air flow will not resultin the abrupt change in the temperature of the thermistors 305 and 306.Therefore, no electric power oscillation will be produced.

MODIFICATION 1

Referring to FIG. 16, a heating device of this modification comprisesthree heat dissipating blocks 301, 302 and 303, and four thermistors305W, 305X, 306W and 306X, in which the thermistors 305W and 305X arealigned side-by-side to each other and positioned between the blocks 301and 302, while the thermistors 306W and 306X are aligned side-by-side toeach other and positioned between the blocks 302 and 303.

The heat dissipating blocks 301 and 303 have flanges 301a, 301b, 303aand 303b, each of which has a U-shaped recess 321 (FIG. 18) formed bydie casting or cutting.

The heat dissipating block 302 in the center has a pair of flanges 302aand 302b protruding laterally outwards therefrom and flush with thesurface of the block 302 in contact with the thermistors 305W and 305X,and another pair of flanges 302c and 302d protruding laterally outwardstherefrom and flush with the other surface held in contact with thethermistors 306W and 306X. Each of the flanges 302a, 302b, 302c and 302dis formed with circular opening 322 (FIG. 16).

The two thermistors 305W and 305X are surrounded by a frame made of anelectrically non-conductive material. The frame 316, as shown in FIG.17, is constituted of a pair of end walls 316a and 316b and a pair oftransverse walls 316c and 316d which are joined together in arectangular shape. A pair of plates 317 and 318 are fixedly attached tothe end walls 316a and 316b, respectively, between the transverse walls316c and 316d. Each of the plates 317 and 318 has an opening 317a, 318aformed at its center for passing a bolt therethrough in a manner whichwill be described later. The thermistors 305W and 305X are accommodatedin a space between the plates 317 and 318 and between the transversewalls 316c and 316d, while the blocks 301 and 302 are fittingly mountedon the frame 316 so as to surround the edge of the blocks 301 and 302 bythe walls 316a, 316b, 316c and 316d.

According to a preferable embodiment, a beam 319 (shown by an imaginaryline) can be extended between the centers of the transverse plates 316cand 316d for separating the opening and defining a space for each of thethermistors 305W and 305X.

It is to be noted that the thickness of the beam 319 and the plates 317and 318 are thinner than that of the thermistors 305W and 305X forensuring the contact between the opposite flat surface of thethermistors and the surface of the heat dissipating blocks.

Similarly, two thermistors 306W and 306X held between the heatdissipating blocks 302 and 303 are surrounded by the frame 316 of thesame type as the above mentioned frame 316. The frame 316 is providednot only to prevent the thermistors from being undesirably shifted, butalso to keep away the dust or small particles from a space between thetwo neighboring thermistors.

According to this modification, the heat dissipating blocks 301, 302 and303 are held together by the use of sets of bolt and nut and supportingmembers as described below.

The supporting member 320, as shown in FIG. 18, is made of anelectrically non-conductive material, such as a resin or a ceramics, andincludes a back plate 320a, support plate 320b perpendicularly extendingfrom an intermediate portion of the back plate 320a, and a cylinder 320cextending from the center of the support plate 320b in parallel to theback plate 320a. A bore 320d is formed through the cylinder 320c andthrough the support plate 320b for inserting a bolt. The end portion ofthe bore 320d adjacent to the support plate 320b is tapered. The backplate 320a has a U-shaped recess formed at its end portion.

When the thermistors 305W and 305X are held in position between theblocks 301 and 302 in the manner described above, each of the plates ofthe frames 316, for example, the plates 318 of the frame 316 (FIG. 19)is sandwiched between two neighboring flanges 301a and 302a with theU-shaped recess 321 and the openings 318a and 322 being aligned witheach other. In this example, the flanges 301a and 302a are joinedtogether in the following steps. First, the supporting member 320 ismounted in such a manner that the cylinder 320c thereof is insertedthrough the U-shaped recess 321 of the flange 301a and through theopening 318a of the frame 316. Then, a bolt 324a mounted with a washer324b is inserted through the bore 320d and through the opening 322 ofthe flange 302a. Then, a nut 324c is screwed on the bolt 324a fortightly holding the flange 301a, the plate 318 and the flange 302atogether. Other neighboring flanges are also tightly held together inthe same manner. The contact between the thermistors and thecorresponding blocks can be ensured when the nut 324c is tightened tobent the washer 324b into the tapered end of the box 320d against itsown resiliency.

After all the neighboring flanges have been joined together, each of theback plate 320a of the supporting member 320 extends along the side ofthe heat dissipating block, as shown in FIG. 16. The U-shaped recess320e in each of the supporting members 320 is provided for supportingthe heating device on a base (not shown).

Each of the heat dissipating blocks 301, 302 and 303 has a pair of maleplugs 325a and 325b in a shape of nipple-ended pin and extendingoutwards from their side surfaces for the external electric connection.

When the voltage from a suitable power source (not shown) is appliedbetween the male plug 325a of the block 301 and the male plug 325a ofthe block 302, an electric current flows through the ceramic plates 305aof the thermistors 305W and 305X for generating heat therefrom.Similarly, when the voltage is applied between the male plug 325a of theblock 303 and the male plug 325a of the block 302, an electric currentflows through the ceramic plates 306a of the thermistors 306W and 306Xfor generating heat therefrom. For actuating the thermistors 305W, 305X,306W and 306X to generate heat at the same time, the male plugs 325a ofthe blocks 301 and 303 are interconnected with each other and in turn toone side of the power source, and the male plug 325a of the block 302 isconnected to the other side of the power source. The electricalconnection to the male plugs can be carried out by the use of femaleplug 326, as shown in FIG. 20.

Although the electrical connections mentioned above are carried out bythe use of male plugs 325a positioned on the right-hand side of theheating device, it is possible to use the male plugs 325b positioned onthe left-hand side solely or in combination with the right-hand maleplugs 325a. Furthermore, the male plugs positioned on one side of theheating device can be used for carrying out cascade connection of aplurality of heating units, as will be described in detail later inconnection with FIG. 29.

Referring to FIG. 21, there is shown a modified supporting member 320'which has the back plate 320a protruding outwards from the heatingdevice.

Referring to FIG. 22, there is shown a further modification. In thismodification, the supporting member 320" and the neighboring flanges ofthe heat dissipating blocks are held together by the use of a springmember 327 instead of a set of bolt and nut. The spring member 327 (FIG.23) has a cylindrical tube configuration with its side partly cut offalong its longitudinal side. The supporting member 320" in thismodification is constituted of a surrounding wall 320f of cubic shapeand a back plate 320a extending from one side of the wall 320f. Theneighboring flanges, i.e., flanges 301b and 302b sandwiching the plate318 of the frame 316 are inserted into the square opening formed by wall320f of the supporting member 320" together with the pinched springmember 327.

Referring to FIG. 24, the heat dissipating blocks 301, 302 and 303 inthis modification are held together by two sets of bolt and nut, one oneach side of the heating device. Furthermore, the male plugs, i.e., 325aare formed in L-shape, instead of the shape of nipple ended pin. The endof each of the L-shaped male plugs has a hook 328 for the engagementwith a female plug 326', as shown in FIG. 25.

MODIFICATION 2

Referring to FIGS. 26 and 27, a heating device of this modificationincludes three heat dissipating blocks 301, 302 and 303, and fourthermistors 305W, 305X, 306W and 306X, which are positioned by theframes 316 and are aligned in a similar manner to those thermistorsdescribed above in connection with FIG. 16.

Each of the heat dissipating blocks 301, 302 and 303 has a bore 330formed at each side portion thereof in a direction perpendicular to thethrough-holes 304. When viewed in FIG. 27, the bore 330 on theright-hand side of each block is formed for inserting a bolt 331a whilethe bore 330 on the left-hand side of each block is formed for insertinga bolt 332a.

When the blocks 301, 302 and 303 are combined together, the bores 330 onthe right-hand side of the blocks 301, 302 and 303, are aligned witheach other for receiving a set of bolt 331a and nut 331b, while thebores 330 on the left-hand side of the blocks are aligned with eachother for receiving a set of bolt 332a and nut 332b to tightly hold thethermistors between the blocks. Besides holding the blocks together, thebolts 331a and 332a are provided for the electrical connection betweenthe heat dissipating blocks 301 and 303. For this purpose, a tube 309made of an insulating material is mounted on each of the bolts 331a and332a over a section where the bolt passes through the bore 330 of theblock 302.

Preferably, as shown in FIGS. 26 and 28, each of the bores 330 in theblock 301 has one end remote from the surface held in contact with thethermistors, enlarged in diameter for receiving therein the head portionof the bolt. Similarly, the bores 330 in the block 303 has one endremote from the surface held in contact with the thermistors, enlargedin diameter for receiving therein the nut.

Each of the dissipating blocks 301 and 303 has two openings 329 at itscorner portions in a direction parallel to the through-holes 304 forinserting bolt (not shown) for supporting the heating device on a base(not shown).

The voltage from the power source (not shown) is applied to heatingdevice through the male plugs 325a or 325b of L-shape. These male plugs325a and 325b can be formed by the nipple-ended pins, as shown in FIG.28.

Referring to FIG. 29, there is illustrated a method for combining aplurality of, e.g., two heating devices together by the use ofconnecting members 332. Each of the connecting members 332 isconstituted of two female plugs formed on opposite ends.

MODIFICATION 3

Referring to FIGS. 30 and 31, a heating device of this modificationincludes three heat dissipating blocks 301, 302 and 303, and fourthermistors 305W, 305X, 306W and 306X located in the frames 316 andaligned in a similar manner to those described above in connection withFIG. 16.

According to this modification, each of the heat dissipating blocks hasa pair of nipple-ended pins 325a and 325b protruding laterally outwardsfrom the side surface thereof. Each of the frames 316 is formed by fourwalls joined together in the shape of rectangular and has a pair ofT-shaped wings 336a and 336b extending from its opposite end walls 316aand 316b, respectively.

For supporting the blocks and thermistors together, the heating deviceof this modification employs a U-shaped frame 333 made of anelectrically non-conductive material, such as resin, and constituted ofan elongated bottom plate 333a and two side plates 333b and 333c whichare extending perpendicularly from the opposite ends of the bottom plate333a. Each of the side plates 333b and 333c has an elongated slot 334which extends from an upper edge of the corresponding side plate 333b or333c and terminates adjacent to the bottom plate 333a for receiving thenipple-ended pins and wings. The peripheral edge of the correspondingside plate defining the slot is so recessed or grooved at 335 as tofittingly engage the wings 336a and 336b of the frame 316 when theheating device is assemble in the U-shaped frame 333. It is preferableto form the guide groove 335 in such a manner that its depth is greaterthan half the thickness of the corresponding side plate 333b or 333c.

After the blocks 301, 302 and 303 and the frames 316 locating thethermistors have been installed in the U-shaped frame 333, an elongatedtop plate 337 made of an electrically non-conductive material, such asresin is rigidly mounted on the upper edge portion of the side plates333b and 333c for maintaining the assembled blocks and frames in theframe 333. The connection between the U-shaped frame 333 and the topplate 337 is carried out by four screws 339, each of which is firstinserted into an opening 340 formed at upper edge portion of each of thebifurcated arms constituting the side plates 333b and 333c, and thenthreaded into an opening 338 formed at each end face of a cross-barportion of each of the T-shaped wings 337a and 337b. To prevent theblocks 301, 302 and 303 and frames 316 from being moved up and down inthe U-shaped frame 333 and to tightly hold the thermistors between theblocks, a spring member 341 is provided between the upper plate 337 andthe block 301. According to this embodiment, the spring member 341 ismade of a phosphor bronze plate rolled in the shape of cylinder or bentin the shape of arc.

A projection 342 having an opening 342a extends outwardly from the topplate 337 for attaching the heating device onto a base (not shown). Asimilar projection 343 formed with an opening 343a extends outwardlyfrom the bottom plate 333a of the U-shaped frame for the same purpose.

It is to be noted that each frame 316 for locating the thermistors canbe provided with a separation bar 344 extending between the centers ofthe transverse walls, as shown in FIG. 32.

Furthermore, instead of the employment of the wings 336a and 336b, suchas shown in FIGS. 31 and 32, each frame 316 may have engagement walls345 fast or integral with the respective transverse walls, as shown inFIG. 33. Each of the engagement walls 345 has a width so selected to belarger than the thickness of the frame 316 that a pair of opposedupright wall areas 345a and 345b or 345c and 345d are defined one oneach side of the respective transverse walls. Preferably, for thepurpose of giving an appearance comfortable to look at, the outersurface of each of the engagement walls opposite to the respectivetransverse wall is outwardly curved. When the heating device isassembled, the surface of each heat dissipating block which is held incontact with the thermistors is fittingly held between the facingupright wall areas 345a and 345c and 345b and 345d.

Since the operation of the heating device of this modification issimilar to the heating device described in the above modifications, adetailed description therefor is omitted for the sake of brevity.

MODIFICATION 4

Referring to FIG. 34, a heating device of this modification includesthree heat dissipating blocks 301, 302 and 303, and four thermistors305W, 305X, 306W and 306X located in the frames 316 and aligned in asimilar manner to those thermistors described above in connection withFIG. 16.

Each heat dissipating block has a pair of nipple-ended pins 325a and325b protruding laterally outwards from the side surface thereof. Theheat dissipating block 301 further has a pair of engagement pins 345aand 345b positioned adjacent to the nipple-ended pins 325a and 325b,respectively.

For supporting the blocks and thermistors together, the heating deviceof this modification employs a pair of supporting plates 350a and 350b(FIG. 35) each including an elongated rectangular plate 351 formed withtwo square openings 352a and 352b at the opposite end portions of theelongated plate 351 and three circular openings 352c, 352d and 352ealigned between the square openings 352a and 352b and spaced apredetermined distance from each other. The supporting plates, e.g.,350a further includes a pair of plates 353 and 354 projectingperpendicularly from one surface and opposite end portions,respectively, of the elongated plate 351 with the surface of the plates353 and 354 being aligned with a longitudinal edge of the supportingplate 350. The plates 353 and 354 have circular openings 353a and 354a,respectively, at their center.

The supporting plates 350a and 350b are positioned in face-to-facerelation to each other and are spaced from each other a predetermineddistance which is slightly greater than the longitudinal length of theheat dissipating block so as to support the assembled blocks 301, 302and 303 and frames 316 carrying the thermistors between the supportingplates 350a and 350b. The heat dissipating block 301 is held between theplates 350a and 350b in such a manner that the engagement pins 345a and345b are inserted into the square openings 352a of the plates 350a and350b, respectively, and the nipple-ended pins 325a and 325b are insertedinto the circular openings 352c of the plates 350a and 350b,respectively. Similarly, the nipple-ended pins 325a and 325b of the heatdissipating block 302 are inserted into the circular openings 352d ofthe plates 350a and 350b, respectively, while the nipple-ended pins 325aand 325b of the heat dissipating block 303 are inserted into thecircular openings 352e of the plates 350a and 350b, respectively.

An elongated plate 355 made of an electrically non-conductive materialhas projections 356a and 356b each extending outwardly from respectiveends of the plate 355. The plate 355 is held between the supportingplates 350a and 350b with its one surface facing the heat dissipatingblock 303 in such a manner that the projections 356a and 356b areinserted into the square openings 352b of the supporting plates 350a and350b, respectively. According to a preferable embodiment, the endportion of each projections 356a and 356b is provided with a hook 357,as shown in FIG. 36, which engages with the corresponding square opening352a.

A spring member 358 formed by a corrugated plate is located between theplate 354 and the heat dissipating block 303 for tightly holding thethermistors 305W, 305X, 306W and 306X between the corresponding blocks.

Referring to FIG. 37, each of the nipple-ended pins projecting outwardlyfrom the corresponding circular opening can be mounted with anengagement ring 359 for preventing supporting plates 350a and 350b frombeing separated apart from the blocks before the heating device isattached to the base (not shown).

The attachment of the heating device on the base is carried out byscrews or the like, connecting the plates 353 and the base.

The operation of the heating device in this modification is carried outin a similar manner to the heating device described in the foregoingmodifications.

MODIFICATION 5

Referring to FIG. 38, a heating device of this modification includesthree heat dissipating blocks 301, 302 and 303 and two thermistors 305and 306 which are held between the blocks in a manner similar to theheating device described above in connection with FIG. 15. The heatdissipating block 302 has sheets 360 made of an electricallynon-conductive material attached on each side face thereof. The blocks301, 302 and 303 are binded together by a pair of flexible metal sheets361 and 362 which are interconnected with each other at respectiveopposite ends by sets of bolt 363a and nut 363b for completelysurrounding the blocks. Since the side surfaces of the block 302 havethe insulation sheets 360, and since the metal sheets 361 and 362directly touches the peripheral faces of the blocks 301 and 303, themetal sheets 361 and 362 are electrically in common with the blocks 301and 303. Therefore, one terminal of a power source (not shown) can beconnected to any portion of the metal sheets 361 and 362 and the otherterminal can be connected to the heat dissipating block 302.

For ensuring the rigid contact between the thermistor and thecorresponding blocks, a bonding agent made of an electrically conductivematerial may be deposited at respective areas of contact of thethermistor to the corresponding blocks.

MODIFICATION 6

This modification relates to the pattern of through-holes 304 formed inthe heat dissipating block 301 which has only one surface held incontact with the thermistor, such as heat dissipating blocks 301 and303. Therefore, each of FIGS. 39 to 42 only shows the heat dissipatingblock 301 and corresponding thermistor 305. For facilitating thedescription, the surface of the heat dissipating block 301 which is heldin contact with the thermistor 305 is referred to as a bottom surfaceBS; the surface opposite to the bottom surface BS is referred to as atop surface TS; and left- and right-hand side surfaces of the block 301are referred to as left surface LS and right surface RS, respectively.

Referring to FIG. 39, the through-holes 304 are densely distributed inthe region away from the bottom surface BS than in the region close tothe bottom surface BS, and are aligned in a form of matrix. In thisarrangement, each of the through-holes 304 has a square cross-section.The distribution of the through-holes are described in detail below.

The through-holes in the first row R1 are spaced a distance T₁ from thebottom surface BS. The through-holes in the second row R₂ are spaced adistance t₁ from the first row R₁. In general, the through-holes in theith row R_(i) (i is an integer) is spaced a distance t_(i-1) from thethrough-holes in the (i-1)th row. The through-holes in the last rowR_(n) (n is an integer greater than i) is spaced a distance T₂ from thetop surface TS.

The through-holes in the first column C₁ are spaced a distance D₁ fromthe left surface LS and, the through-holes in the last column C_(m) (mis an integer) are spaced a distance D₂ from the right surface RS. Thetwo neighboring columns, e.g., C₁ and C₂ are spaced a distance d fromeach other. The relation among the distances mentioned above can beexpressed as follows:

    T.sub.1 >t.sub.1 >t.sub.2 >->t.sub.n-1

    T.sub.1 ≈T.sub.2 ≈D.sub.1 ≈D.sub.2

According to the above arrangement, the heat transmitted from thethermistor 305 is first accumulated in the solid block portion at 365between the bottom surface BS and the first row R₁ and is graduallytransmitted towards the top surface TS through peripheral main passagesdefined at 366, 367 and 368 and also through branch passages defined at369 between the two neighboring columns.

Since the heat capacity is generally in relation to the volume of amaterial accumulating the heat, it is understood that the heat capacityis greatest in the solid block portion 365 and is decreased towards thetop surface TS. The heat accumulated in the portion 365 is thenaccumulated in the main passages 366, 367 and 368. Thereafter, theaccumulated heat is transmitted through the branch passages and isreleased to the fluid to heat the fluid passing through thethrough-holes 304. As described above, since the heat is transmittedfrom a portion of high heat capacity to a portion of low heat capacity,the fluid passing through the through-holes 304 can be uniformly heated.

Referring to FIG. 40, there is shown another pattern of through-holes304 each having a circular cross-section. The through-holes 304 arealigned in a form of matrix and the through-holes aligned in column arein pairs. More particularly, the distance t between the (2n-1)th columnand the 2nth column is smaller than the distance T between the 2nthcolumn and the (2n+1)th column. According to this pattern, a mainpassages 370 is formed between the pairs of columns and a branch passage371 is formed between the columns in the pair. The through-holes alignedin two neighboring rows are spaced a predetermined distance which isapproximately equal to the distance t. The through-holes in the firstrow are spaced a distance T₁ from the bottom surface BS to form thesolid block portion 365 thereat. The peripheral main passages 366, 367and 368 are formed around the through-holes.

The relation among the distances mentioned above can be expressed asfollows:

    T.sub.1 >T>t

According to the above arrangement, the heat emitted from the thermistoris transmitted through the main passages 365, 366, 367, 368 and 370, andthen through the branch passages 371. Therefore, the fluid passingthrough the through-holes can be heated with high efficiency.

Referring to FIG. 41, there is shown a further pattern of through-holes.In this arrangement, the number of through-holes to be formed in one rowis increased with the increase of number of rows so that thethrough-holes are densely distributed in the region away from the bottomsurface than in the region close to the bottom surface BS.

Instead of increasing the number of through-holes, the diameter d of thethrough-holes to be formed in one row can be increased with the increaseof the number of the rows, as shown in FIG. 42.

In the through-hole arrangements shown in FIGS. 41 and 42, the distancebetween the two neighboring rows can be equal, as shown in FIG. 40, orcan be varied in the manner described above in connection with FIG. 39.

Although this modification is described under the heading of "Example3", the through-hole patterns described above can be applied to the heatdissipating blocks in the other embodiments.

It is to be understood that, while the invention has been described inconjunction with certain specific embodiments, the scope of the presentinvention is not to be limited thereby except as defined in the appendedclaims.

What is claimed is:
 1. A heating device for heating fluid comprising, incombination:a heat dissipating means having a plurality of through-holesdefined therein, said through-holes extending in a first direction andpermitting air to be heated to flow along said first direction only; aheating unit including a thermistor element having a positivetemperature coefficient characteristic, said thermistor element beingadapted to generate heat when electric power is applied thereto, saidheat dissipating means being mounted on said heating unit in thermalconductive relation therewith such that heat generated by said heatingunit is transmitted to the heat dissipating means to heat fluid flowingthrough the through-holes, said thermistor element including a pair ofelectrodes between which heating current flows, the location of saidelectrodes being such that said current flows in a second directionsubstantially perpendicular to said first direction; and means forconnecting said heat dissipating means and the heating unit together. 2.A heating device as claimed in claim 1, wherein said heat dissipatingmeans comprises first and second heat dissipating blocks disposed onopposite sides of said heating unit and in thermal contact therewith. 3.A heating device as claimed in claim 2, wherein said heating unit has athickness as measured in said second direction and wherein each of saidheat dissipating blocks has a recess formed therein, said recess havinga depth as measured in said second direction which is smaller thanone-half the thickness of said heating unit for receiving said heatingunit therein.
 4. A heating device for heating a fluid passing throughsaid heating device, said heating device comprising:at least one heatingunit including a thermistor plate having a positive temperaturecoefficient characteristic, and first and second electrodes deposited,respectively, on opposite flat surfaces of said thermistor plate, saidthermistor element generating heat when electric power is appliedbetween said first and second electrodes; first and second heatdissipating blocks, each of said blocks having first and second opposingfaces, at least one side flat face extending between respective edges ofthe first and second opposing faces, and a plurality of through-holesextending between said first and second faces in a first directionparallel to each other, said through-holes permitting air to be heatedto flow along said first direction only, said first and secondelectrodes being so located than when electric power is applied to saidelectrodes, current flows between said electrodes along a seconddirection substantially perpendicular to said first direction; saidheating unit being sandwiched between said first and second heatdissipating blocks such that said first electrode of said heating unitis held in contact with a portion of said side flat face of said firstheat dissipating block and said second electrode of said heating unit isheld in contact with a portion of said side flat face of said secondheat dissipating block with said through-holes of said first and secondheat dissipating blocks being aligned in the same direction such thatheat generated by said heating unit is transmitted to said first andsecond heat dissipating blocks to heat fluid flowing through thethrough-holes; frame means made of electrically non-conductive materialand surrounding side surfaces of said heating unit so as to preventdirect contact between said fluid and said heating unit; connectingmeans for connecting said first and second heat dissipating blocks, saidheating unit and said frame means; and terminal means for permittingelectric power to be supplied across said first and second electrodes.5. A heating device as claimed in claim 2, wherein each of said firstand second heat dissipating blocks is made of metal.
 6. A heating deviceas claimed in claim 4, wherein said frame means includes a wall meansmade of electrically non-conductive material for surrounding the edgesbetween said first face and said one side flat face and between saidsecond face and said one side flat face of said first and second heatdissipating blocks.
 7. A heating device as claimed in claim 3, whereinsaid connecting means comprises at least one fastener including a nutand bolt, said fastener being coupled between said first and second heatdissipating blocks in a manner which maintains said heat dissipatingblocks insulated from each other.
 8. A heating device as claimed inclaim 7, wherein said fastener is connected to first and second flangeportions extending from said first and second heat dissipating blocks,respectively, each flange portion being extending outwardly from itsrespective heat dissipating block in a direction parallel to said sideflat face of said respective heat dissipating block.
 9. A heating deviceas claimed in claim 7, wherein said fastener extends through boresformed in said first and second heat dissipating blocks, said boresextending in a direction perpendicular to said side flat faces.
 10. Aheating device as claimed on claim 5, wherein said terminal meanscomprises first and second terminal members connected to said first andsecond heat dissipating blocks, respectively.
 11. A heating device asclaimed in claim 10, wherein each of said terminal members is anipple-ended projection extending from its respective heat dissipatingblock.
 12. A heating device as claimed in claim 10, wherein each of saidterminal members is an L-shaped projection extending from of itsrespective heat dissipating block.
 13. A heating device as claimed inclaim 4, wherein each of said through-holes is hexagonal incross-section.
 14. A heating device as claimed in claim 4, wherein eachof said through-holes is circular in cross-section.
 15. A heating deviceas claimed in claim 4, wherein each of said through-holes is rectangularin cross-section.
 16. A heating device as claimed in claim 4, whereinthe density of the distribution of said through-holes of each of saidfirst and second heat dissipating blocks increases with the distance ofsaid through-holes from said heating unit.
 17. A heating device asclaimed in claim 4, wherein said through-holes of each of said first andsecond heat dissipating blocks are larger in diameter in a first regionlocated a first distance from said heating unit than in a second regionlocated in a second distance from said heating unit, said first distancebeing greater than said second distance.
 18. A heating device as claimedin claim 4, wherein said through-holes of each of said first and secondheat dissipating blocks form a matrix, the column of said matrix beingperpendicular to said side flat faces, the distance between the (2n-1)thcolumn and 2nth column being smaller than the distance between 2nthcolumn and (2n+1)th column, n being an integer.
 19. A heating device forheating fluid comprising:first and second heating units each including athermistor plate having a positive temperature coefficientcharacteristic, and first and second electrodes located, respectively,on opposite flat surfaces of said thermistor plate, each of said firstand second heating units generating heat when electric power is appliedbetween said first and second electrodes; first, second and third heatdissipating blocks; each of said first and third heat dissipating blockshaving first and second opposing faces, at least one side flat faceextending between respective edges of the first and second opposingfaces, and a plurality of through-holes extending between said first andsecond faces in parallel to each other; said second heat dissipatingblock having first and second opposite faces, first and second side flatfaces opposed to each other and extending between said first and secondopposed faces, and a plurality of through-holes extending between saidfirst and second opposed faces in parallel to each other; said firstheating unit being sandwiched between said first and second heatdissipating blocks such that said first electrode of said first heatingunit is held in contact with a portion of said side flat face of saidfirst heat dissipating block and said second electrode of said firstheating unit is held in contact with a portion of said first side flatface of said second heat dissipating block; said second heating unitbeing sandwiched between said second and third heat dissipating blockssuch that said first electrode of said second heating unit is held incontact with a portion of said second side flat face of said second heatdissipating block and said second electrode of said second heating unitis held in contact with a portion of said side flat face of said thirdheat dissipating block; said through-holes of said first, second andthird heat dissipating blocks being aligned in the same direction;whereby heat generated by said first and second heating units istransmitted to said first, second and third heat dissipating blocks tothereby heat a fluid flowing through the through-holes; first and secondframe means made of electrically non-conductive material and surroundingside surfaces of said first and second heating units, respectively, soas to prevent direct contact between said fluid and said heating units;connecting means for connecting said first, second and third heatdissipating blocks, said first and second heating units, and said firstand second frame means together; and terminal means for permittingelectric power to be applied across said first and second electrodes ofeach of said first and second heating units, said through-holes allextending along a first direction and adapted to permit air to be heatedto flow along said first direction only, said first and secondelectrodes of each heating unit being so located that current flowsbetween said electrodes in a second direction substantiallyperpendicular to said first direction.
 20. A heating device as claimedin claim 19, wherein the height of said second heat dissipating block asmeasured in a direction perpendicular to said first and second side flatfaces is greater than the height of each of said first and third heatdissipating blocks as measured in a direction perpendicular to saidfirst and second side flat faces.
 21. A heating device as claimed inclaim 20, wherein said frame means and said heating unit each have athickness as measured in a direction perpendicular to said first andsecond side flat faces of said second heat dissipating block and whereinsaid thickness of said frame means is no greater than the thickness ofsaid heating unit.
 22. A heating device as claimed in claim 4, whereinsaid frame means and said heating unit each have a thickness as measuredin a direction perpendicular to said one side flat face of said firstand second heat dissipating blocks and wherein said thickness of saidframe means is no greater than the thickness of said heating unit.